Retrofit of School HVAC System in Phoenix
Engineering Case Study
Case Study 2: Retrofit of School HVAC System in Phoenix
Scenario: A 1970s-era elementary school in Phoenix, AZ underwent an energy efficiency retrofit funded by a DOE RECAP grant. The scope included replacing aging sheet metal ductwork in corridor ceilings and classrooms. Constraints included summer shutdown window (only 3 weeks between academic years), budget cap limiting sealant labor hours, and mandatory compliance with IECC 2021 §M1403.4 (requiring ≤ 6% duct leakage for conditioned air systems). The engineer prioritized targeted sealing—only where leakage exceeded thresholds—to stay within budget and timeline.
Given Data:
- Measured leakage flow rate = 612 cfm
- Total system flow rate = 8,600 cfm
- Test pressure = 1.0 in. w.g. (per IECC allowance for existing duct retrofits tested at reduced pressure)
Calculation:
- Leakage Percentage = (612 ÷ 8600) × 100 ≈ 7.12%
- Duct Leakage Class determination:
- At 1.0 in. w.g., SMACNA allows direct comparison to Class II threshold (≤6%) only if test pressure is normalized or adjusted per SMACNA Section 4.3. However, IECC §M1403.4 explicitly permits testing at 1.0 in. w.g. and enforces the 6% limit regardless of pressure.
- Since 7.12% > 6%, the system fails the IECC requirement.
- Note: SMACNA Class II at 1.0 in. w.g. is not formally defined—so classification defaults to “Non-Compliant” under code, not a formal SMACNA class.
Result and Decision: The engineer directed focused aerosol-based duct sealing (AeroSeal®) on the main supply trunk downstream of the AHU—where infrared imaging identified thermal bypass paths—and retested. Post-sealing: 498 cfm leakage → 5.79% → compliant. Final documentation included pre/post test reports, pressure decay curves, and third-party verification for grant compliance.
Lesson: When testing at non-standard pressures (e.g., 1.0 in. w.g. for retrofits), prioritize code-mandated percentage thresholds over SMACNA class labels—SMACNA classes assume standardized test pressures (typically 1.5 or 2.5 in. w.g.), and misapplying them can mask noncompliance.